If you are using a mobile browser, try using the eBay app or else request the desktop site version of the page with your mobile browser. Or try another browser.
Details about requesting the desktop site version of the page in various browsers can be found here:
If all else fails, try reading your messages on ebay.ca instead of ebay.com.
If you have your messages sent to your email address, you may be able to read and respond that way.
You should be able to go to a user's feedback or profile page and compose a new message there using the "Contact" link if you are unable to respond to a message. If prompted for an item number, try selecting "not about an item" if the item number is old or you get an error message.
